The Urban Heat Island Effect

Urban areas do interesting things to our atmosphere, the most noteworthy of which is the urban heat island effect.

If you have ever travelled from a large city to a rural area or vice versa, you may have noticed that the city is usually warmer – sometimes by several degrees. This is a documented phenomenon all over the globe, and although they’re most noticeable at night they can also be observed during the day.

Numerous studies have been done by atmospheric scientists to document and understand the urban heat island effect. Heat islands have been noticed around cities like Barcelona, Saint Louis, Denver, Boston, and many more; in general, the larger a city is, the more intense the heat island effect becomes.

Heat islands require calm, clear conditions to become strong. They can still be detectable on cloudy and breezy days, but not as strongly if at all. Daytime temperatures in urban areas can be anywhere from 1 to 7 degrees warmer than in surrounding rural areas, and nighttime temperatures can be 2 to 5 degrees warmer.

One of the key reasons why the urban heat island exists is that urban and rural areas absorb sunlight differently. Asphalt is good at absorbing and holding onto incoming sunlight as compared to trees and cropland. Continually, parking lots and roadways take little to no time to heat up in comparison to natural areas. Large, paved areas also provide less shade than forests (for example), so surface temperatures tend to be a lot hotter there.

Urban areas also have less water as compared to rural areas. Water is good at absorbing energy from the sun, and it can absorb a significant amount of energy before changing temperature as compared to non-water substances. That’s why big lakes and oceans tend to be cooler than land-locked areas, and also why large, dry urban areas often bake on summer days.

Buildings are also partly to blame for heat islands in large cities. Air flow is often greatly disrupted by skyscrapers, warehouses, and closely spaced buildings of any size. Heat quickly builds in any place where cooler air can’t be drawn in, mainly where winds are blocked such as in areas with narrow streets and downwind of tall buildings.

In addition to these somewhat natural causes, there is also an important direct human cause to mention: machines. Car engines, air conditioners, and factories – all part of normal and essential activities – can make our cities hotter.

There are ways that the heat island effect can be mitigated. Adding trees and gardens can help a lot, especially when they are added to roofs. Making sure that our roofs and pavement will reflect sunlight can also help – the less solar energy our cities absorb, the cooler they will be. Because light materials reflect heat, some cities have been 'lightening' streets by covering them with a reflective gray coating. Studies have shown changes like this can save close to $100 million per year in energy costs.

Source: EPA, NASA

Story Image: Tianjin, China as seen from above. Image by 畅 苏 from Pixabay.